If one Gmail shortcut isn't enough to satisfy you, here is the mother of them all: shift + ?. What does it do? It tells you every shortcut in Gmail so you'll never go shortcut-hungry again.
That's pretty much it. When you press shift + ? in Gmail you'll get a shortcut cheat sheet on top of your inbox or whatever you've loaded up into your account. (Bonus fake tip: when you're looking at disgusting, inappropriate images at work, shift + ? will cover them up...sort of, because the overlay is translucent. It's super fast and sort of effective!) For a demonstration, check out the video above.
Note: For those of you who haven't enabled shortcuts in Gmail, just pop into your settings panel and click on General (if it isn't already selected. You should find an option to enable keyboard shortcuts here, letting you in on all the timesaving fun.